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Versions: (draft-klemets-avt-rtp-vc1) 00 01 02 03 04 05 06 RFC 4425

   Internet Engineering Task Force
   Internet Draft                                            A. Klemets
   Document: draft-ietf-avt-rtp-vc1-01.txt                    Microsoft
   Expires: April 2006                                     October 2005


                RTP Payload Format for Video Codec 1 (VC-1)

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Copyright Notice

      Copyright (C) The Internet Society (2005).

Abstract

   This memo specifies an RTP payload format for encapsulating Video
   Codec 1 (VC-1) compressed bit streams, as defined by the proposed
   Society of Motion Picture and Television Engineers (SMPTE) standard,
   SMPTE 421M.  SMPTE is the main standardizing body in the motion
   imaging industry and the proposed SMPTE 421M standard defines a
   compressed video bit stream format and decoding process for
   television.




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Table of Contents

   1. Introduction...................................................2
      1.1 Conventions used in this document..........................3
   2. Definitions and abbreviations..................................3
   3. Overview of VC-1...............................................4
      3.1 VC-1 bit stream layering model.............................5
      3.2 Bit-stream Data Units in Advanced profile..................5
      3.3 Decoder initialization parameters..........................6
      3.4 Ordering of frames.........................................7
   4. Encapsulation of VC-1 format bit streams in RTP................8
      4.1 Access Units...............................................8
      4.2 Fragmentation of VC-1 frames...............................8
      4.3 Time stamp considerations..................................9
      4.4 Random Access Points......................................10
      4.5 Removal of HRD parameters.................................10
      4.6 Repeating the Sequence Layer header.......................11
      4.7 Signaling of MIME format parameters.......................11
      4.8 MIME "mode=1" parameter...................................12
      4.9 MIME "mode=3" parameter...................................12
   5. RTP Payload Format syntax.....................................13
      5.1 RTP header usage..........................................13
      5.2 AU header syntax..........................................13
      5.3 AU Control field syntax...................................14
   6. RTP Payload format parameters.................................16
      6.1 Media Type Registration...................................16
      6.2 Mapping of MIME parameters to SDP.........................19
   7. Security Considerations.......................................20
   8. IANA Considerations...........................................21
   9. References....................................................21
      9.1 Normative references......................................21
      9.2 Informative references....................................21

1.
  Introduction

   The bit stream syntax for compressed video in Video Codec 1 (VC-1)
   format is defined by SMPTE 421M [1]. SMPTE 421M also specifies
   constraints that must be met by VC-1 conformant bit streams, and it
   specifies the complete process required to decode the bit stream.
   However, it does not specify the VC-1 compression algorithm, thus
   allowing for different ways to implement a VC-1 encoder.

   The VC-1 bit stream syntax has three profiles. Each profile has
   specific bit stream syntax elements and algorithms associated with
   it.  Depending on the application in which VC-1 is used, some
   profiles may be more suitable than others.  For example, the Simple
   profile is designed for low bit rate Internet streaming and for
   playback on devices that can only handle low complexity decoding.
   The Advanced profile is designed for broadcast applications, such as


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   digital TV, HD DVD or HDTV.  The Advanced profile is the only VC-1
   profile that supports interlaced video frames.

   Section 2 defines the abbreviations used in this document.  Section 3
   provides a more detailed overview of VC-1.  Sections 4 and 5 define
   the RTP payload format for VC-1, and section 6 defines the MIME and
   SDP parameters for VC-1.  See section 7 for security considerations.

1.1
   Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [2].

2.
  Definitions and abbreviations

   This document uses the definitions in SMPTE 421M [1].  For
   convenience, the following terms from SMPTE 421M are restated here:

   B-picture: A picture that is coded using motion compensated
   prediction from past and/or future reference fields or frames.  A B-
   picture cannot be used for predicting any other picture.

   Bit-stream data unit (BDU): A unit of the compressed data which may
   be parsed (i.e., syntax decoded) independently of other information
   at the same hierarchical level.  A BDU can be, for example, a
   sequence layer header, an entry-point header, a frame, or a slice.

   Encapsulated BDU (EBDU): A BDU which has been encapsulated using the
   encapsulation mechanism described in Annex E of SMPTE 421M [1], to
   prevent emulation of the start code prefix in the bit stream.

   Entry-point: A point in the bit stream that offers random access.

   frame: A frame contains lines of spatial information of a video
   signal.  For progressive video, these lines contain samples starting
   from one time instant and continuing through successive lines to the
   bottom of the frame.  For interlaced video, a frame consists of two
   fields, a top field and a bottom field.  One of these fields will
   commence one field period later than the other.

   interlace: The property of frames where alternating lines of the
   frame represent different instances in time.  In an interlaced frame,
   one of the fields is meant to be displayed first.

   I-picture: A picture coded using information only from itself.





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   level: A defined set of constraints on the values which may be taken
   by the parameters (such as bit rate and buffer size) within a
   particular profile.  A profile may contain one or more levels.

   P-picture: A picture that is coded using motion compensated
   prediction from past reference fields or frames.

   picture: For progressive video, a picture is identical to a frame,
   while for interlaced video, a picture may refer to a frame, or the
   top field or the bottom field of the frame depending on the context.

   profile: A defined subset of the syntax of VC-1, with a specific set
   of coding tools, algorithms, and syntax associated with it.  There
   are three VC-1 profiles: Simple, Main and Advanced.

   progressive: The property of frames where all the samples of the
   frame represent the same instance in time.

   random access: A random access point in the bit stream is defined by
   the following guarantee: If decoding begins at this point, all frames
   needed for display after this point will have no decoding dependency
   on any data preceding this point, and are also present in the
   decoding sequence after this point.  A random access point is also
   called an entry-point.

   sequence: A coded representation of a series of one or more pictures.
   In VC-1 Advanced profile, a sequence consists of a series of one or
   more entry-point segments, where each entry-point segment consists of
   a series of one or more pictures, and where the first picture in each
   entry-point segment provides random access.  In VC-1 Simple and Main
   profiles, the first picture in each sequence is an I-picture.

   slice: A consecutive series of macroblock rows in a picture, which
   are encoded as a single unit.

   start codes (SC): 32-bit codes embedded in that coded bit stream that
   are unique, and identify the beginning of a BDU.  Start codes consist
   of a unique three-byte Start Code Prefix (SCP), and a one-byte Start
   Code Suffix (SCS).

3.
  Overview of VC-1

   The VC-1 bit stream syntax consists of three profiles: Simple, Main,
   and Advanced.  The Simple and Main profiles are designed for
   relatively low bit rate applications.  For example, the maximum bit
   rate supported by the Simple profile is 384 kbps.  To help achieve
   high compression efficiency, certain features such as non-square
   pixels and support for interlaced pictures, are only included in the
   Advanced profile.


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   The maximum bit rate supported by the Advanced profile is 135 Mbps,
   making it suitable for nearly lossless encoding of HDTV signals.
   Only the Advanced profile supports carrying user-data (meta-data) in-
   band with the compressed bit stream.  The user-data can be used for
   closed captioning support, for example.

   Of the three profiles, only the Advanced profile allows codec
   configuration parameters, such as the picture aspect ratio, to be
   changed through in-band signaling in the compressed bit stream.

   For each of the profiles, a certain number of "levels" have been
   defined.  Unlike a "profile", which implies a certain set of features
   or syntax elements, a "level" is a set of constraints on the values
   of parameters in a profile, such as the bit rate or buffer size.  The
   VC-1 Simple profile has two levels, the Main profile has three, and
   the Advanced profile has five levels.  See Annex D of SMPTE 421M [1]
   for a detailed list of the profiles and levels.

3.1
   VC-1 bit stream layering model

   The VC-1 bit stream is defined as a hierarchy of layers.  This is
   conceptually similar to the notion of a protocol stack of networking
   protocols.  The outermost layer is called the sequence layer.  The
   other layers are entry-point, picture, slice, macroblock and block.

   In the Simple and Main profiles, a sequence in the sequence layer
   consists of a series of one or more coded pictures.  In the Advanced
   profile, a sequence consists of one or more entry-point segments,
   where each entry-point segment consists of a series of one or more
   pictures, and where the first picture in each entry-point segment
   provides random access.  A picture is decomposed into macroblocks.  A
   slice comprises one or more contiguous rows of macroblocks.

   The entry-point and slice layers are only present in the Advanced
   profile.  In the Advanced profile, the start of each entry-point
   layer segment indicates a random access point.  In the Simple and
   Main profiles each I-picture is a random access point.

   Each picture can be coded as an I-picture, P-picture, skipped
   picture, BI-picture, or as a B-picture.  These terms are defined in
   section 2 of this document and in section 4.12 of SMPTE 421M [1].

3.2
   Bit-stream Data Units in Advanced profile

   In the Advanced profile only, each picture and slice is byte-aligned
   and is considered a Bit-stream Data Unit (BDU).  A BDU is defined as
   a unit that can be parsed (i.e., syntax decoded) independently of
   other information in the same layer.


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   The beginning of a BDU is signaled by an identifier called Start Code
   (SC).  Sequence layer headers and entry-point headers are also BDUs
   and thus can be easily identified by their Start Codes.  See Annex E
   of SMPTE 421M [1] for a complete list of Start Codes.  Note that
   blocks and macroblocks are not BDUs and thus do not have a Start Code
   and are not necessarily byte-aligned.

   The Start Code consists of four bytes.  The first three bytes are
   0x00, 0x00 and 0x01. The fourth byte is called the Start Code Suffix
   (SCS) and it is used to indicate the type of BDU that follows the
   Start Code.  For example, the SCS of a sequence layer header (0x0F)
   is different from the SCS of an entry-point header (0x0E).  The Start
   Code is always byte-aligned and is transmitted in network byte order.

   To prevent accidental emulation of the Start Code in the coded bit
   stream, SMPTE 421M defines an encapsulation mechanism that uses byte
   stuffing.  A BDU which has been encapsulated by this mechanism is
   referred to as an Encapsulated BDU, or EBDU.

3.3
   Decoder initialization parameters

   In the VC-1 Advanced profile, the sequence layer header contains
   parameters that are necessary to initialize the VC-1 decoder.  These
   parameters apply to all entry-point segments until the next
   occurrence of a sequence layer header in the coded bit stream.

   The parameters in the sequence layer header include the Advanced
   profile level, the dimensions of the coded pictures, the aspect
   ratio, interlace information, the frame rate and up to 31 leaky
   bucket parameter sets for the Hypothetical Reference Decoder (HRD).

   Section 6.1 of SMPTE 421M [1] provides the formal specification of
   the sequence layer header.

   Each leaky bucket parameter set for the HRD specifies a peak
   transmission bit rate and a decoder buffer capacity.  The coded bit
   stream is restricted by these parameters.  The HRD model does not
   mandate buffering by the decoder.  Its purpose is to limit the
   encoder's bit rate fluctuations according to a basic buffering model,
   so that the resources necessary to decode the bit stream are
   predictable.  The HRD has a constant-delay mode and a variable-delay
   mode.  The constant-delay mode is appropriate for broadcast and
   streaming applications, while the variable-delay mode is designed for
   video conferencing applications.

   Annex C of SMPTE 421M [1] specifies the usage of the hypothetical
   reference decoder for VC-1 bit streams.  A general description of the
   theory of the HRD can be found in [7].


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   The concept of an entry-point layer applies only to the VC-1 Advanced
   profile.  The presence of an entry-point header indicates a random
   access point within the bit stream.  The entry-point header specifies
   current buffer fullness values for the leaky buckets in the HRD.  The
   header also specifies coding control parameters that are in effect
   until the occurrence of the next entry-point header in the bit
   stream.  See Section 6.2 of SMPTE 421M [1] for the formal
   specification of the entry-point header.

   Neither a sequence layer header nor an entry-point header is defined
   for the VC-1 Simple and Main profiles.  For these profiles, decoder
   initialization parameters MUST be conveyed out-of-band from the coded
   bit stream.  Section 4.7 of this document specifies how the
   parameters are conveyed by this RTP payload format.

3.4
   Ordering of frames

   Frames are transmitted in the same order in which they are captured,
   except if the presence of B-pictures has been indicated in the
   decoder initialization parameters.  In the latter case, the frames
   are reordered by the VC-1 encoder such that the frames that the B-
   pictures depend on are transmitted first.  This is referred to as the
   coded order of the frames.

   When the presence of B-pictures has been indicated, the decoder is
   required to buffer one picture.  When an I-picture or a P-picture is
   received, the picture is not displayed until the next I- or P-picture
   is received.  However, B-pictures are displayed immediately.  These
   rules are stated in section 5.4 in SMPTE 421M [1].

   Figure 1 illustrates the timing relationship between the capture of
   frames, their coded order, and the display order of the decoded
   frames.  The figure shows that the display of frame P4 is delayed
   until frame P5 is received, while frames B2 and B3 are displayed
   immediately.


   Capture:        |I0  P1  B2  B3  P4  ...
                   |
   Coded order:    |        I0  P1  P4  B2  B3  P5  ...
                   |
   Display order:  |            I0  P1  B2  B3  P4  ...
                   |
                   |+---+---+---+---+---+---+---+------> time
                    0   1   2   3   4   5   6   7

   Figure 1.  Frame reordering when B-pictures are indicated.



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4.
  Encapsulation of VC-1 format bit streams in RTP

4.1
   Access Units

   Each RTP packet contains an integral number of application data units
   (ADUs).  For VC-1 format bit streams, an ADU is equivalent to one
   Access Unit (AU), as defined in this section.  Figure 2 shows the
   layout of an RTP packet with multiple AUs.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+- .. +-+-+-+-+
   | RTP     | AU(1) | AU(2) |     | AU(n) |
   | Header  |       |       |     |       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+- .. +-+-+-+-+

   Figure 2.  RTP packet structure.

   Access Units MUST be byte-aligned.  Each Access Unit MUST start with
   the AU header defined in section 5.2, and is followed by a variable
   length payload.

   The AU payload MUST contain data belonging to exactly one VC-1 frame.

   The following rules apply to the contents of each AU payload when the
   VC-1 Advanced profile is used:

   - The AU payload MUST contain VC-1 bit stream data in EBDU format
     (i.e., the bit stream must use the byte-stuffing encapsulation
     mode defined in Annex E of SMPTE 421M [1].)

   - The AU payload MAY contain multiple EBDUs, e.g., a sequence layer
     header, an entry-point header, a frame header and multiple slices
     and the associated user-data.  (However, all slices and their
     corresponding macroblocks MUST belong to the same video frame.)

   - The AU payload MUST start at an EBDU boundary, except when the AU
     payload contains a fragmented frame, in which case the rules in
     section 4.2 apply.

   If the data in an AU (EBDUs in the case of Advanced profile and frame
   in the case of Simple and Main) does not end at an octet boundary, up
   to 7 zero-valued padding bits MUST be added to achieve octet-
   alignment.

4.2
   Fragmentation of VC-1 frames

   Each AU payload SHOULD contain a complete VC-1 frame.  However, if
   this would cause the RTP packet to exceed the MTU size, the frame
   SHOULD be fragmented into multiple AUs to avoid IP-level


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   fragmentation.  When an AU contains a fragmented frame, this MUST be
   indicated by setting the FRAG field in the AU header as defined in
   section 5.3.

   AU payloads that do not contain a fragmented frame, or that contain
   the first fragment of a frame, MUST start at an EBDU boundary if
   Advanced profile is used.  In this case, for Simple and Main
   profiles, the AU payload MUST begin with the start of a frame.

   If Advanced profile is used, AU payloads that contain a fragment of a
   frame other than the first fragment, SHOULD start at an EBDU
   boundary, such as at the start of a slice.

   However, slices are only defined for the Advanced profile, and are
   not always used.  Blocks and macroblocks are not BDUs (have no Start
   Code) and are not byte-aligned.  Therefore, it may not always be
   possible to continue a fragmented frame at an EBDU boundary.

   If a RTP packet contains an AU with the last fragment of a frame,
   additional AUs SHOULD NOT be included in the RTP packet.

   If the PTS Delta field in the AU header is used, each fragment of a
   frame MUST have the same presentation time.  If the DTS Delta field
   in the AU header is used, each fragment of a frame MUST have the same
   decode time.

4.3
   Time stamp considerations

   Video frames MUST be transmitted in the coded order.  Coded order
   implies that no frames are dependent on subsequent frames, as
   discussed in section 3.4.  The RTP timestamp field MUST be set to the
   presentation time of the video frame contained in the first AU in the
   RTP packet.  The presentation time is equivalent to the sampling
   instant of the frame.

   Each AU header may optionally specify the decode time of video frame
   contained in the AU.  If the decode time is not specified, the RTP
   receiver can approximate it by the frame's presentation time, after
   taking frame reordering into account.  Frame reordering can be
   handled by an algorithm similar to the one illustrated in Figure 1 in
   section 3.4.  The algorithm requires buffering of only one frame.

   Knowing if the stream will contain B-pictures helps the decoder
   allocate resources more efficiently, as the encoder will not reorder
   any frames.  In that case, the buffering of one frame as described in
   section 3.4 is not necessary.  Avoiding this buffer reduces the end-
   to-end delay, which may be important for interactive applications.
   For Advanced profile, B-pictures are assumed to be present by



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   default.  If the coded bit stream never contains B-pictures, this can
   be indicated using the "bpic" MIME parameter defined in section 6.1.

   For Simple and Main profiles, the presence of B-pictures is indicated
   by a non-zero value for the MAXBFRAMES field in STRUCT_C decoder
   initialization parameter.  STRUCT_C conveyed in the MIME "config"
   parameter, which is defined in section 6.1.

4.4
   Random Access Points

   The entry-point header contains information that is needed by the
   decoder to decode the frames in that entry-point segment.  This means
   that in the event of lost RTP packets the decoder may be unable to
   decode frames until the next entry-point header is received.

   The first frame after an entry-point header is a random access points
   into the coded bit stream.  The Simple and Main profiles do not have
   entry-point headers, so for those profiles each I-picture is a random
   access point.

   To allow the RTP receiver to detect that a RTP packet which was lost
   contained a random access point, this RTP payload format defines a
   field called "RA Count".  This field is present in every AU, and its
   value is incremented for every random access point.  For additional
   details, see the definition of "RA Count" in section 5.2.

   To make it easy to determine if a AU contains a random access point,
   this RTP payload format also defines a bit called the "RA" flag in
   the AU Control field.  This bit is set to 1 only on those AU's that
   contain a random access point.  The RA bit is defined in section 5.3.

4.5
   Removal of HRD parameters

   The sequence layer header of the Advanced profile may include up to
   31 leaky bucket parameter sets for the Hypothetical Reference Decoder
   (HRD).  Each leaky bucket parameter set specifies a possible peak
   transmission bit rate (HDR_RATE) and a decoder buffer capacity
   (HRD_BUFFER).  (See section 3.3 for additional discussion about the
   HRD.)

   If the actual peak transmission rate is known by the RTP sender, the
   RTP sender MAY remove all leaky bucket parameter sets except for the
   one corresponding to the actual peak transmission rate.

   For each leaky bucket parameter set in the sequence layer header,
   there is also parameter in the entry-point header that specifies the
   initial fullness (HRD_FULL) of the leaky bucket.




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   If the RTP sender has removed any leaky bucket parameter sets from
   the sequence layer header, then for any removed leaky bucket
   parameter set, it MUST also remove the corresponding HRD_FULL
   parameter in the entry-point header.

   Removing leaky bucket parameter sets, as described above, may
   significantly reduce the size of the sequence layer headers and the
   entry-point headers.

4.6
   Repeating the Sequence Layer header

   To improve robustness against loss of RTP packets, it is RECOMMENDED
   that if the sequence layer header changes, it should be repeated
   frequently in the bit stream.  In this is case, it is RECOMMENDED
   that the number of leaky bucket parameters in the sequence layer
   header and the entry point headers be reduced to one, as described in
   section 4.5.  This will help reduce the overhead caused by repeating
   the sequence layer header.

   Note that any data in the VC-1 bit stream, including repeated copies
   of the sequence header itself, must be accounted for when computing
   the leaky bucket parameter for the HRD.  (See section 3.3 for a
   discussion about the HRD.)

   Note that if the value of TFCNTRFLAG in the sequence layer header is
   1, each picture header contains a frame counter field (TFCNTR).  Each
   time the sequence layer header is inserted in the bit stream, the
   value of this counter MUST be reset.

   To allow the RTP receiver to detect that a RTP packet which was lost
   contained a new sequence layer header, the AU Control field defines a
   bit called the "SL" flag.  This bit is toggled when a sequence layer
   header is transmitted, but only if that header is different from the
   most recently transmitted sequence layer header.  The SL bit is
   defined in section 5.3.

4.7
   Signaling of MIME format parameters

   When this RTP payload format is used with SDP, the decoder
   initialization parameters described in section 3.3 MUST be signaled
   in SDP using the MIME parameters specified in section 6.1.  Section
   6.2 specifies how to map the MIME parameters to SDP.

   When the Advanced profile is used, the decoder initialization
   parameters MAY be changed by inserting a new sequence layer header or
   an entry-point header in the coded bit stream.

   Note that the sequence layer header specifies the encoding level, the
   maximum size of the coded pictures and possibly also the frame rate.


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   Thus, if the sequence layer header changes, the new header supersedes
   the values of the MIME parameters "level", "width", "height" and
   "framerate".

4.8
   MIME "mode=1" parameter

   In certain applications using the Advanced profile, the sequence
   layer header never changes.  This MAY be signaled with the MIME
   parameter "mode=1". (The "mode" parameter is defined in section 6.1.)
   The "mode=1" parameter serves as a "hint" to the RTP receiver that
   all sequence layer headers in the bit stream will be identical.  If
   "mode=1" is signaled and a sequence layer header is present in the
   coded bit stream, then it MUST be identical to the sequence layer
   header specified by the MIME "config" parameter.

   Since the sequence layer header never changes in "mode=1", the RTP
   sender MAY remove it from the bit stream.  Note, however, that if
   that if the value of TFCNTRFLAG in the sequence layer header is 1,
   each picture header contains a frame counter field (TFCNTR).  This
   field is reset each time the sequence layer header occurs in the bit
   stream.  If the RTP sender chooses to remove the sequence layer
   header, then it MUST ensure that the resulting bit stream is still
   compliant with the VC-1 specification (e.g., by adjusting the TFCNTR
   field, if necessary.)

4.9
   MIME "mode=3" parameter

   In certain applications using the Advanced profile, both the sequence
   layer header and the entry-point header never change.  This MAY be
   signaled with the MIME parameter "mode=3".  The same rules apply to
   "mode=3" as for "mode=1", described in section 4.8.  Additionally, if
   "mode=3" is signaled, then the RTP sender MAY "compress" the coded
   bit stream by not including sequence layer headers and entry-point
   headers in the RTP packets.

   The RTP receiver MUST "decompress" the coded bit stream by re-
   inserting the entry-point headers prior to delivering the coded bit
   stream to the VC-1 decoder.  The sequence layer header does not need
   to be decompressed by the receiver, since it never changes.

   If "mode=3" is signaled and the RTP receiver receives a complete AU
   or the first fragment of an AU, and the RA bit is set to 1 but the AU
   does not begin with an entry-point header, then this indicates that
   entry-point header has been "compressed".  In that case, the RTP
   receiver MUST insert an entry-point header at the beginning of the
   AU.  When inserting the entry-point header, the RTP receiver MUST use
   the one that was specified by the MIME "config" parameter.




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5.
  RTP Payload Format syntax

5.1
   RTP header usage

   The format of the RTP header is specified in RFC 3550 [3] and is
   reprinted in Figure 3 for convenience.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |V=2|P|X|  CC   |M|     PT      |       sequence number         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           timestamp                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           synchronization source (SSRC) identifier            |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |            contributing source (CSRC) identifiers             |
     |                             ....                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 3.  RTP header according to RFC 3550

   With the exception of the fields listed below, the RTP header fields
   are used as defined in RFC 3550 and by the RTP profile in use.

   Marker bit (M): 1 bit
           This bit is set to 1 if the RTP packet contains an Access
           Unit containing a complete VC-1 frame, or the last fragment
           of a VC-1 frame.

   Payload type (PT): 7 bits
           This document does not assign a RTP payload type for this RTP
           payload format. The assignment of a payload type has to be
           performed either through the RTP profile used or in a dynamic
           way.

   Timestamp: 32 bits
           The RTP timestamp is set to the presentation time of the VC-1
           frame in the first Access Unit.
           A 90 kHz clock rate MUST be used.

5.2
   AU header syntax

   The Access Unit header consists of a one-byte AU Control field, the
   RA Count field and 3 optional fields.  The structure of the AU header
   is illustrated in
   Figure 4.




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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AU     | RA    |  AUP  | PTS   | DTS   |
   |Control| Count |  Len  | Delta | Delta |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4.  Structure of AU header.

   AU Control: 8 bits
           The usage of the AU Control field is defined in section 5.3.

   RA Count: 8 bits
           Random Access Point Counter.  This field is a binary modulo
           256 counter.  The value of this field, MUST be incremented by
           1, each time an AU is transmitted where the RA bit in the AU
           Control field is set to 1.  The initial value of this field
           is undefined and MAY be chosen randomly.

   AUP Len: 16 bits
           Access Unit Payload Length.  Specifies the size, in bytes, of
           the payload of the Access Unit.  The field does not include
           the size of the AU header itself.  The field MUST be included
           in each AU header in an RTP packet, except for the last AU
           header in the packet.

   PTS Delta: 32 bits
           Presentation time delta.  Specifies the presentation time of
           the frame as a 2's complement offset (delta) from the
           timestamp in the RTP header of this RTP packet.  The PTS
           Delta field MUST use the same clock rate as the timestamp
           field in the RTP header.
           This field SHOULD NOT be included in the first AU header in
           the RTP packet, because the RTP timestamp field specifies the
           presentation time of the frame in the first AU.

   DTS Delta: 32 bits
           Decode time delta.  Specifies the decode time of the frame as
           a 2's complement offset (delta) from the timestamp in the RTP
           header of this RTP packet.  The DTS Delta field MUST use the
           same clock rate as the timestamp field in the RTP header.

5.3
   AU Control field syntax

   The structure of the 8-bit AU Control field is shown in Figure 5.








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     0    1    2    3    4    5    6    7
   +----+----+----+----+----+----+----+----+
   |  FRAG   | RA | SL | LP | PT | DT | R  |
   +----+----+----+----+----+----+----+----+

   Figure 5.  Syntax of AU Control field.

   FRAG: 2 bits
           Fragmentation Information.  This field indicates if the AU
           payload contains a complete frame or a fragment of a frame.
           It MUST be set as follows:
           0: The AU payload contains a fragment of a frame other than
           the first or last fragment.
           1: The AU payload contains the first fragment of a frame.
           2: The AU payload contains the last fragment of a frame.
           3: The AU payload contains a complete frame (not fragmented.)

   RA: 1 bit
           Random Access Point indicator.  This bit MUST be set to 1 if
           the AU contains a frame that is a random access point.  In
           the case of Simple and Main profiles, any I-picture is a
           random access point.
           In the case of Advanced profile, the first frame after an
           entry-point header is a random access point.
           Note that if entry-point headers are not transmitted at every
           random access point, this MUST be indicated using the MIME
           parameter "mode=3".

   SL: 1 bit
           Sequence Layer Counter.  This bit MUST be toggled, i.e.,
           changed from 0 to 1 or from 1 to 0, if the AU contains a
           sequence layer header and if it is different from the most
           recently transmitted sequence layer header.  Otherwise, the
           value of this bit must be identical to the value of the SL
           bit in the previous AU.
           The initial value of this bit is undefined and MAY be chosen
           randomly.
           The bit MUST be 0 for Simple and Main profile bit streams or
           if the sequence layer header never changes.

   LP: 1 bit
           Length Present.  This bit MUST be set to 1 if the AU header
           includes the AUP Len field.

   PT: 1 bit
           PTS Delta Present.  This bit MUST be set to 1 if the AU
           header includes the PTS Delta field.

   DT: 1 bit


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           DTS Delta Present.  This bit MUST be set to 1 if the AU
           header includes the DTS Delta field.

   R: 1 bit
           Reserved.  This bit MUST be set to 0 and MUST be ignored by
           receivers.

6.
  RTP Payload format parameters

6.1
   Media Type Registration

   The media subtype for VC-1 is allocated from the standards tree.  The
   top-level media type under which this payload format is registered is
   'video'.

   The receiver MUST ignore any unrecognized parameter.

   Media type: video

   Media subtype: vc1

   Required parameters:

         profile:
           The value is a decimal number indicating the VC-1 profile.
           The following values are defined:
           0: Simple profile.
           1: Main profile.
           3: Advanced profile.

         config:
           The value is a base16 [5] (hexadecimal) representation of an
           octet string that expresses the decoder initialization
           parameters.  Decoder initialization parameters are mapped
           onto the base16 octet string in an MSB-first basis.  The
           first bit of the decoder initialization parameters MUST be
           located at the MSB of the first octet.  If the decoder
           initialization parameters are not multiple of 8 bits, in the
           last octet up to 7 zero-valued padding bits MUST be added to
           achieve octet alignment.

           For the Simple and Main profiles, the decoder initialization
           parameters are STRUCT_C, as defined in Annex J of SMPTE 421M
           [1].

           For the Advanced profile, the decoder initialization
           parameters are a sequence layer header directly followed by
           an entry-point header.  The two headers MUST be in EBDU
           format, meaning that they must include their Start Codes and


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           must use the encapsulation method defined in Annex E of SMPTE
           421M [1].

         width:
           The value is a decimal number specifying the maximum
           horizontal size of the coded picture in pixels.

           Note: When Advanced profile is used, this parameter only
           applies while the sequence layer header specified in the
           config parameter is in use.

         height:
           The value is a decimal number specifying the maximum vertical
           size of the coded picture in pixels.

           Note: When Advanced profile is used, this parameter only
           applies while the sequence layer header specified in the
           config parameter is in use.

         bitrate:
           The value is a decimal number specifying the peak
           transmission rate of the coded bit stream.  The number does
           not include RTP overhead.

           Note: When Advanced profile is used, this parameter only
           applies while the sequence layer header specified in the
           config parameter is in use.

         buffer:
           The value is a decimal number specifying the leaky bucket
           size, B, in milliseconds, required to contain a stream
           transmitted at the transmission rate specified by the bitrate
           parameter.  This parameter is defined in the hypothetical
           reference decoder model for VC-1, in Annex C of SMPTE 421M
           [1].

           Note: When Advanced profile is used, this parameter only
           applies while the sequence layer header specified in the
           config parameter is in use.

   Optional parameters:

         level:
           The value is a decimal number specifying the level of the
           encoding profile.
           For Advanced profile, valid values are 0 to 4, which
           correspond to levels L0 to L4, respectively.  For Simple and
           Main profiles, the following values are defined:
           1: Low Level


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           2: Medium Level
           3: High Level (only valid for Main profile)

           This parameter does not have a default value.

           Note: When Advanced profile is used, this parameter only
           applies while the sequence layer header specified in the
           config parameter is in use.

         framerate:
           The value is a decimal number specifying the number of frames
           per second, multiplied by 1000.  For example, 29.97 frames
           per second is represented as 29970.

           This parameter does not have a default value.  If the
           parameter is not specified, the frame rate can be determined
           from the level of the encoding profile, if it is known.

           Note: When Advanced profile is used, this parameter only
           applies while the sequence layer header specified in the
           config parameter is in use.

         bpic:
           This parameter signals if B-pictures may be present when the
           Advanced profile is used.  If this parameter is present, and
           B-pictures may be present in the coded bit stream, this
           parameter MUST be equal to 1.
           If B-pictures will never be present in the coded bit stream,
           even if the sequence layer header changes, this parameter
           SHOULD be present and its value SHOULD be equal to 0.

           If this parameter is not specified, a value of 1 MUST be
           assumed.

         mode:
           The value is a decimal number specifying the use of the
           sequence layer header and the entry-point header.  This
           parameter is only used for Advanced profile.  The following
           values are defined:
           0: Both the sequence layer header and the entry-point header
           may change, and changed headers will be included in the RTP
           packets.
           1: The sequence layer header specified in the config
           parameter never changes.
           3: The sequence layer header and the entry-point header
           specified in the config parameter never change.  Entry-point
           headers MAY not be included in the Access Units.  Each Access
           Unit that has the RA bit set to 1 contains a random access
           point even if an entry-point header is not included in the


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           Access Unit.  If an entry-point header is not included at a
           random access point, then the RTP receiver MUST insert the
           entry-point header into the VC-1 bit stream prior to
           delivering the bit stream to the VC-1 decoder.

           If the mode parameter is not specified, a value of 0 MUST be
           assumed.  The mode parameter SHOULD be specified if modes 1
           or 3 apply to the VC-1 bit stream.

   Encoding considerations:
           This media type is framed and contains binary data.

   Security considerations:
           See Section 7 of this document.

   Interoperability considerations:
           None.

   Published specification:
           This payload format specification.

   Applications which use this media type:
           Multimedia streaming and conferencing tools.

   Additional Information:
           None.

   Person & email address to contact for further information:
           Anders Klemets <anderskl@microsoft.com>
           IETF AVT working group.

   Intended Usage:
           COMMON

   Restrictions on usage:
           This media type depends on RTP framing, and hence is only
           defined for transfer via RTP [3].

   Authors:
           Anders Klemets

   Change controller:
           IETF Audio/Video Transport Working Group delegated from the
           IESG.

6.2
   Mapping of MIME parameters to SDP

   The information carried in the media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)


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   [4].  If SDP is used to specify sessions using this payload format,
   the mapping is done as follows:

   o The media name in the "m=" line of SDP MUST be video (the media
     type).

   o The encoding name in the "a=rtpmap" line of SDP MUST be vc1 (the
     media subtype).

   o The clock rate in the "a=rtpmap" line MUST be 90000.

   o The REQUIRED parameters "profile", "config", "width", "height",
     "bitrate" and "buffer" MUST be included in the "a=fmtp" line of
     SDP.
     These parameters are expressed as a MIME media type string, in the
     form of a semicolon separated list of parameter=value pairs.

   o The OPTIONAL parameters "level", "framerate", "bpic" and "mode",
     when present, MUST be included in the "a=fmtp" line of SDP.
     These parameters are expressed as a MIME media type string, in the
     form of a semicolon separated list of parameter=value pairs:

         a=fmtp:<dynamic payload type> <parameter
         name>=<value>[,<value>][; <parameter name>=<value>]

   o Any unknown parameters to the device that uses the SDP MUST be
     ignored.  For example, parameters defined in later specifications
     MAY be copied into the SDP and MUST be ignored by receivers that
     do not understand them.

   An example of media representation in SDP is as follows (Simple
   profile, Medium level):

   m=video 49170 RTP/AVP 98
   a=rtpmap:98 VC1/90000
   a=fmtp:98 profile=0;level=2;width=352;height=288;framerate=15000;
   bitrate=384000;buffer=2000;config=4e291800

7.
  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [4], and in any appropriate RTP profile.  This implies
   that confidentiality of the media streams is achieved by encryption;
   for example, through the application of SRTP [6].

   A potential denial-of-service threat exists for data encodings using
   compression techniques that have non-uniform receiver-end
   computational load.  The attacker can inject pathological RTP packets


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   into the stream that are complex to decode and that cause the
   receiver to be overloaded.  VC-1 is particularly vulnerable to such
   attacks, because it is possible for an attacker to generate RTP
   packets containing frames that affect the decoding process of many
   future frames.  Therefore, the usage of data origin authentication
   and data integrity protection of at least the RTP packet is
   RECOMMENDED; for example, with SRTP [6].

   Note that the appropriate mechanism to ensure confidentiality and
   integrity of RTP packets and their payloads is very dependent on the
   application and on the transport and signaling protocols employed.
   Thus, although SRTP is given as an example above, other possible
   choices exist.

8.
  IANA Considerations

   IANA is requested to register the media subtype name "vc1" for the
   media type "video" as specified in section 6.1 of this document.

9.
  References

9.1
   Normative references

   [1] Proposed SMPTE 421M, "VC-1 Compressed Video Bitstream Format and
       Decoding Process", www.smpte.org.
   [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
   [3] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
       "RTP: A Transport Protocol for Real-Time Applications", STD 64,
       RFC 3550, July 2003.
   [4] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
       RFC 2327, April 1998.
   [5] Josefsson, S., Ed., "The Base16, Base32, and Base64 Data
       Encodings", RFC 3548, July 2003.

9.2
   Informative references

   [6] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
       Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
       3711, March 2004.
   [7] Ribas-Corbera, J., Chou, P.A., and S.L. Regunathan, "A
       generalized hypothetical reference decoder for H.264/AVC", IEEE
       Transactions on Circuits and Systems for Video Technology, August
       2003.

Author's Addresses

   Anders Klemets
   Microsoft Corp.


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   1 Microsoft Way
   Redmond, WA 98052
   USA
   Email: anderskl@microsoft.com

Acknowledgements

   Thanks to Shankar Regunathan for pointing out errors in the initial
   draft of this document.

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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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